1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image-forming apparatus using it and to the optical scanning apparatus suitably applicable to the image-forming apparatus, for example, such as laser beam printers, digital copiers, etc. involving the electrophotographic process, which is constructed to converge light emitted from a light source means, by a first imaging optical system, reflectively deflect the converged light by a polygon mirror as a deflecting means, and optically scan a surface to be scanned, through a second imaging optical system to record image information thereon. More particularly, the invention relates to the optical scanning apparatus capable of controlling focus movement amounts to small values on the surface to be scanned, even with variation in ambient temperature and/or with change in the initial operating wavelength of the light from the light source means, so as to be able to keep variation little in the spot size of the beam, thereby always providing good images, and to the image-forming apparatus using it.
2. Related Background Art
FIG. 5 is a schematic diagram to show the principal part of a light scanning optical system in a conventional optical scanning apparatus.
In the conventional optical scanning apparatus illustrated in FIG. 5, the light modulated based on image information and emitted from the light source means 1 is incident to a first imaging optical system L1 comprised of a collimator lens 2, a stop 3, and a cylindrical lens 4.
In this apparatus the light from the light source means 1 is converted into nearly parallel light by the collimator lens 2, the nearly parallel light is limited by the aperture stop 3, and the light is then incident to the cylindrical lens 4 having a predetermined refractive power only in the sub-scanning section.
The nearly parallel light incident to the cylindrical lens 4 emerges in the nearly parallel light state in the main scanning section as it is.
The light is converged in the sub-scanning section to be focused as an almost linear image on a deflective reflection facet 5a of a polygon mirror 5 (deflecting means). The light reflectively deflected by the polygon mirror 5 is guided through a second imaging optical system L2 with the fxcex8 characteristic onto a surface to be scanned 7 (a surface of a photosensitive drum), and the polygon mirror 5 is rotated to optically scan the surface to be scanned 7 (the surface of the photosensitive drum) to record the image information.
For example, Japanese Patent Application Laid-Open No. H06-118346 suggests a modification of the optical scanning system of the structure illustrated in FIG. 5, in which focus movement on the surface to be scanned 7 is corrected by applying a diffraction optical element to part of the optics.
In the above application the focal length of a resin condenser lens is set to a value capable of canceling out change in the focal length of a Fresnel lens due to variation in the oscillation wavelength of the laser diode and change in the focal length of the Fresnel lens due to variation in temperature.
Japanese Patent Application Laid-Open No. H10-333070 describes that the scanning optical apparatus has a first optical system for guiding the light emitted from the light source means, to the deflecting means and a second optical system for focusing the light deflected by the deflecting means, on the surface to be scanned and a diffraction optical element is provided in part of at least one optical system of the first and second optical systems whereby aberration variation in the sub-scanning direction of the scanning optical system due to environmental variation (temperature variation) is corrected by change in power of the diffraction optical element and variation in the wavelength of the light source means.
In the both cases, the focus movement caused by the optical elements other than the diffraction optical element is canceled by the power change of the diffraction optical element, making use of the property that the diffraction optical element greatly changes the power, depending upon the operating wavelength.
Incidentally, factors causing the focus movement on the surface to be scanned include, for example, dispersion or variation of the oscillation wavelength of the light from the light source means and change in ambient temperature, but mechanisms of focus variation are greatly different from each other.
The former is nothing but the focus variation affected only by the power change of the optical systems due to the variation in the operating wavelength, whereas the latter is the focus variation caused by combination of refractive index change of materials and positional deviation of the optical elements, change in the oscillation wavelength of the light from the light source means, etc. due to the change in the ambient temperature.
Since these hold independently, the aforementioned suggestions were not always satisfactory to these factors.
Specifically, the light scanning optical system in Japanese Patent Application Laid-Open No. 06-118346 failed to include consideration to the focus movement caused by the environmental change. For example, it is focus movement due to positional variation of the collimator lens and focus movement caused by change in refractive powers of the collimator lens, cylindrical lens, fxcex8 lens, etc. due to the change in the oscillation wavelength of the light from the light source means. For that reason, the optical system was one that was not always able to make good correction for focus movement amounts with change of ambient temperature.
Since the operating wavelength largely varies depending upon the light source means under practical use, focus movement amounts caused by the change of the operating wavelength are unignorable in the light scanning optical systems using the diffraction optical element that largely changes the power of the optical system with change in the operating wavelength. However, the above application failed to take it into consideration.
Japanese Patent Application Laid-Open No. H10-333070 gave consideration to the focus movement caused by the change of ambient temperature, including the above factors of focus movement, but failed to take the variation in the operating wavelength (initial wavelength) of the light source means into consideration.
The present invention has been accomplished in order to solve the above problems and an object of the invention is thus to provide an optical scanning apparatus capable of always forming good images while controlling variation to small values in the spot size of the beam on the surface to be scanned, by reducing the focus movement amounts even on the occasion of simultaneous occurrence of the change in the oscillation wavelength of the light from the light source means due to dispersion of the oscillation wavelength of the light from the light source means and/or due to the change of the ambient temperature, and the change in refractive indexes of the materials of the optical systems due to the change of ambient temperature, and to provide an image-forming apparatus using it.
An optical scanning apparatus according to one aspect of the invention is an optical scanning apparatus comprising light source means, a first imaging optical system for converging light emitted from the light source means, deflecting means for deflecting the light from the first imaging optical system, a second imaging optical system for scanning a surface to be scanned, with the light deflected by the deflecting means, and at least one refraction optical element and one diffraction optical element in the first imaging optical system or in the second imaging optical system,
wherein a power of said diffraction optical element is set to a third power between a first power and a second power, where the first power is a power that the diffraction optical element has when focus movement on the surface to be scanned, caused by the refraction optical element with a change of an oscillation wavelength of the light from the light source means, can be canceled by a power change of the diffraction optical element and the second power is a power that the diffraction optical element has when focus movement on the surface to be scanned, caused by the refraction optical elements with a change of ambient temperature, can be canceled by a power change of the diffraction optical element.
In the optical scanning apparatus according to another aspect of the invention, said diffraction optical element has the power in the sub-scanning direction.
In the optical scanning apparatus according to another aspect of the invention, said diffraction optical element is provided in said first imaging optical system.
In the optical scanning apparatus according to another aspect of the invention, said diffraction optical element is placed on a surface closest to said deflecting means in said first imaging optical system.
In the optical scanning apparatus according to another aspect of the invention, said first imaging optical system comprises a cylindrical lens and the diffraction optical element is provided on one surface of said cylindrical lens.
In the optical scanning apparatus according to another aspect of the invention, where a longitudinal magnification of said second imaging optical system in the sub-scanning direction is as (times), a focal length fcl (mm) of said cylindrical lens satisfies the following equation:
fclxe2x89xa6500/xcex1s.
In the optical scanning apparatus according to another aspect of the invention, said cylindrical lens includes no position adjusting means for adjusting the position in the optical-axis direction.
The optical scanning apparatus according to another aspect of the invention comprises a third imaging optical system for converging the light deflected by said deflecting means and guiding the light into light detecting means,
wherein said first imaging optical system comprises a cylindrical lens, said third imaging optical system comprises an imaging lens having a power at least in the main scanning direction, and said cylindrical lens and said imaging lens are integrally formed.
In the optical scanning apparatus according to another aspect of the invention, the following equation is satisfied:
|dxcex94Sxe2x80x94T|xe2x89xa7|dxcex94S_xcex| if |dxcex94STxe2x80x94xcex|xe2x89xa7|dxcex94Sxcexxe2x80x94T|, or
|dxcex94Sxe2x80x94T|xe2x89xa6|dxcex94S_xcex| if |dxcex94STxe2x80x94xcex| less than |dxcex94Sxcexxe2x80x94T|,
where dxcex94Sxcex_T is a focus movement amount with an ambient temperature change when the power of said diffraction optical element is the first power; dxcex94ST_xcex is a focus movement amount with a change of an initial operating wavelength of said light source means when the power of said diffraction optical element is the second power; dxcex94S_T is a focus movement amount with the ambient temperature change and dxcex94S_xcex is a focus movement amount with the change of the initial operating wavelength of said light source means when the power of said diffraction optical element is the third power.
In the optical scanning apparatus according to another aspect of the invention, the elements are set so that a focus movement amount with a change of 1 nm in the operating wavelength is not more than 0.3 mm.
An image-forming apparatus according to a further aspect of the present invention is an image-forming apparatus comprising the scanning optical apparatus as set forth, a photosensitive body placed on said surface to be scanned, a developing unit for developing an electrostatic latent image formed on said photosensitive body with the light under scanning by said scanning optical apparatus, into a toner image, a transfer unit for transferring said toner image developed, onto a transfer medium, and a fixing unit for fixing the toner image transferred, on the transfer medium.
Another image-forming apparatus according to a further aspect of the invention is an image-forming apparatus comprising the scanning optical apparatus as set forth, and a printer controller for converting code data supplied from an external device, into an image signal and supplying the image signal to said scanning optical apparatus.